Improvement of impact strength and toughness of polyamide resins has been made the subject matter of research and development over a considerable period of time. The tendency of articles molded of polyamide resins to crack or break upon impact, in a brittle fashion, imposes a serious limitation on the ability to make use of such molded articles, especially at low temperatures.
The endeavors to improve impact strength and toughness have led to the blending of the polyamides, with a variety of additives including rubbers and other modified and unmodified resins with various degrees of success. One such recent endeavor is described in the Epstein U.S. Pat. No. 4,174,358, issued Nov. 13, 1979, wherein polyamide resins are blended with random copolymers selected to adhere to the polyamide resin.
The aforementioned copending application seeks to overcome some of the deficiencies of the cited Epstein patent by blending the polyamide resin with a low molecular weight, saturated ethylene-mono-olefin copolymer rubber, and preferably an ethylene-propylene copolymer rubber (EPM) which has been grafted to provide labile cross-links with an unsaturated polycarboxylic acid or anhydride and preferably maleic anhydride. The described reversibly cross-linked EPM copolymer rubber acquires the characteristic of a higher molecular weight solid material that enables easy handling and shipment in a solid state, and disruption of cross-links when blended at high temperature with the polyamide base resin at the station of use whereby the EPM components reverts to low molecular weight when in the blend thereby to enhance its utility as a toughening agent and impact improver of the matrix polyamide resin composition.